In the lithographic technology for producing semiconductors, capability of forming more detailed patterns is demanded in accordance with an increase in the degree of integration. Wavelength reduction of the exposure light source is indispensable for miniaturization of patterns. At present, lithography using a krypton fluoride (KrF) excimer laser (wavelength: 248 nm) is acquiring a mainstream position. In addition, lithography with line and space width of 100 nm or less using an argon fluoride (ArF) excimer laser (wavelength: 193 nm) is being put into practice. Lithographic technologies using other short wavelength radiations such as a fluorine dimer (F2) excimer laser (wavelength: 157 nm), extreme ultraviolet radiation (EUV), X-rays, and electron beams are also in the stage of development.
In these semiconductor lithographic technologies, resist coatings for forming patterns on substrates by utilizing the properties of the resist of changing the solubility in an alkaline developer by the action of an acid after transferring a pattern profile onto the substrates, as well as other various coatings on the upper layer or under layer of the resist coatings are used. In the resist polymers used in semiconductor lithography, there are a negative-tone resist polymer of which the solubility in an alkaline developer decreases by the action of an acid generated by exposure to radiation and a positive-tone resist polymer of which the solubility in an alkaline developer increases by the action of such an acid.
Of these, the positive-tone resist polymer comprises a repeating unit having a structure with a non-polar substituent which dissociates by the action of an acid and produces a polar group soluble in an alkali developer and a repeating unit having a polar group for increasing adhesion to a semiconductor substrate as essential components and, optionally, another repeating unit having a polar or non-polar substituent for adjusting the solubility in a resist solvent or an alkaline developer. As the repeating unit having a polar group for providing adhesiveness to a semiconductor substrate, hydroxylstyrenes are mainly used when a KrF excimer laser is used as a radiation source, for example. When an ArF excimer laser is used, the use of (meth)acrylate having a polar group has been studied, because hydroxystyrenes absorb light with a wavelength of 193 nm.
As specific examples of such a positive-tone resist polymer used for KrF excimer laser, resist polymers in which a (meth)acrylic acid monomer is combined with a styrene monomer (e.g. Patent Documents 1-4), polymers in which a part of hydroxystyrenes is protected by acetal (e.g. Patent Documents 5-8), and the like can be given. As resist polymers used for ArF excimer laser, polymers of (meth)acrylic acid monomer having a lactone structure (e.g. Patent Documents 9-10), and the like are known.
Resist patterns are formed by semiconductor lithography in the following manner. First, a resist composition solution, prepared by dissolving a resist polymer, a photoacid generator, and, optionally, additives such as an acid diffusion inhibitor in a solvent for coating film formation, is applied to a semiconductor substrate by spin coating or the like. The coating is prebaked to remove the solvent, thereby obtaining a thin film of which the main component is a resist polymer. The thin film is irradiated with light through a mask pattern to cause an acid to be generated in the irradiated part, optionally followed by heating. Since the irradiated part of the positive-tone resist polymer becomes soluble in alkali, a resist pattern can be formed by washing off the irradiated part with water after developing with an alkaline aqueous solution, followed by drying. On the other hand, in the case of the negative-tone resist polymer, in which the irradiated part becomes insoluble in alkali, a resist pattern can be formed by washing off the non-irradiated part with water after developing using an alkaline aqueous solution, followed by drying.
As the solvent for forming a coating used in the resist composition, usually a solvent with a boiling point of 140° C. or more under atmospheric pressure can be used. A part of this solvent remains in the thin film without being completely removed by prebaking. If any impurities other than the solvent for coating film formation remain in the thin film, problems such as change of the lithographic characteristics and volatilization of the solvent during the irradiation process occur, resulting in damage to high precision irradiation equipment. For this reason, there is a demand for removing as much of these impurities as possible from the composition for semiconductor lithography.
As the impurities other than the above-mentioned solvents for coating-film formation, in addition to compounds used in or originating from production of the resist polymer such as unreacted monomers, polymerization initiators, chain transfer agents, coupling products of these, polymerization solvents, purified solvents, water, and the like, compounds originating from the outside environment can be given.
Of these, impurities originating from the polymerization reaction such as unreacted monomers, polymerization initiators, chain transfer agents, coupling products of these, and polymerization solvents can be removed in a purification step after polymerization.
However, impurities having a boiling point lower than the boiling point of the solvent for coating-film formation under atmospheric pressure such as a purification solvent used in the purification step, water introduced during the purification step or operations after the purification step, and the like have conventionally been removed in a vacuum drying step in which the resist polymer is dried using a stirring-type, rotation-type, shelf-type, or spray-type vacuum drier. Resist polymers have therefore been supplied in the form of a powder.
However, because the impurities such as a solvent used in the purification step and water are incorporated in the resist polymer and can be removed only with difficulty, the resist polymers must be heated at as high a temperature and for as long a period of time as possible in order to remove such impurities in the vacuum drying step. The resist polymers containing repeating units which are decomposed and become alkali-soluble by the action of an acid, however, are easily decomposed by heating. A part of the acid-decomposable repeating units are decomposed during the vacuum drying step requiring heating at a high temperature for a long period of time by the catalytic action of acidic substances, such as a very small amount of carboxylic acid, carboxylic acid halide, hydrogen halide, and the like that originate from the raw materials or are produced during the production process, or by impurities such as water and alcohols originating from the raw materials or introduced during the production process.
For these reasons, a process for producing resist polymers containing impurities only to a minimal extent and being capable of efficiently removing solvents used for purification and impurities introduced after the purification step without decomposing a part of acid-decomposable repeating units has been desired.    [Patent document 1] Japanese Patent Application Laid-open No. 59-045439    [Patent document 2] Japanese Patent Application Laid-open No. 5-113667    [Patent document 3] Japanese Patent Application Laid-open No. 7-209868    [Patent document 4] Japanese Patent Application Laid-open No. 11-065120    [Patent document 5] Japanese Patent Application Laid-open No. 62-115440    [Patent document 6] Japanese Patent Application Laid-open No. 4-219757    [Patent document 7] Japanese Patent Application Laid-open No. 3-223860    [Patent document 8] Japanese Patent Application Laid-open No. 4-104251    [Patent document 9] Japanese Patent Application Laid-open No. 9-073173    [Patent document 10] Japanese Patent Application Laid-open No. 10-239846    [Patent document 11] Japanese Patent Application Laid-open No. 2001-125269    [Patent document 12] WO 9931157 International Publication pamphlet    [Patent document 13] Japanese Patent Application Laid-open No. 10-186662    [Patent document 14] Japanese Patent Application Laid-open No. 64-10239